Differences in the mains voltages in the USA and Europe increase the production costs, as product differentiation is required. For years, therefore, it has been known to make frequency converters, which can be connected to different mains voltages, see for example U.S. Pat. No. 4,656,571.
However, this gives rise to a problem, when a frequency converter is connected to a mains voltage, which is lower than the maximum nominal mains voltage, for which the frequency converter is designed to handle. In this case the current in the unit will be increased in order to maintain the same electrical power output to the motor. The increased current may damage the frequency converter. To avoid this, the frequency converter can be designed for the high current, but this is an expensive solution, which will also require more room, because, for example, the intermediary circuit coil gets bigger. Thus, protection of the frequency converter is required.
Reduction of the current flowing in a frequency converter is known from EP 0 431 563 B1, however, in another connection. When the load, that is, the motor which is here used in a compressor for an HVAC system, consumes too much current, a control device reduces the output frequency of the inverter. The amplitude of the current is detected by a current measurement on the mains side, that is, before the frequency converter. When the output frequency is reduced, the current flowing to the rectifier from the mains is reduced, and thus the installation in private homes is protected against over current. However, the frequency converter described is not intended for connection to one of several different mains voltages.
A mains voltage self-adapting frequency converter is described in the U.S. Pat. No. 4,656,571 mentioned above. In front of the rectifier a voltage detector is placed, which gives information to a control device about the amplitude of the mains voltage. The control device comprises a memory element, in which a table for every possible mains voltage is stored. Each table again contains a number of U/f relations, known per se as the relation between the motor voltage and motor frequency applied by the inverter. To obtain an optimum motor operation, this relation must be kept constant. There are tables containing the U/f relations for mains voltages of 100V, 115V, 200V and 230V, and the output power of the frequency converter is regulated to a constant value by selecting the suitable U/f relations. In this solution, it is endeavoured to keep the intermediary circuit voltage constant, and for this purpose the input circuit contains a switch arrangement, which switches to voltage doubling mode when the unit is connected to 115V, whereas the switching device remains unchanged when connecting the unit to 230V. Thus, the motor receives the same voltage, but this system has the disadvantage that with a 115V mains voltage the input circuit has to sink a higher current. The complete input circuit has to be dimensioned for the lowest mains voltage, meaning that with the higher mains voltages the components will be substantially overdimensioned. A further problem with this design is that non-standard voltages are not considered. It is thus unclear, how the control device will handle a voltage supply drop from 230V to 170V. A stepless operation is not described.
Self-adaptation of a frequency converter should not be limited to adapting to the mains voltages. Also, adaptation of current limits is of interest. Today, a frequency converter typically contains three current limits, the upper limit being a short-circuiting protection, which is activated at a current of 300% of the nominal frequency converter current, and has a response time of 1 to 2 μs. This limit is activated when earthing or a short-circuit appears between the phase windings. The second limit is the hardware limit, which is typically at a current of 220% with a response time of 15 μs. The hardware limit is an expression of the maximum current, which semiconductors and coils of the frequency converter can stand. The short-circuit limit and the hardware limit are realised with electronic components, whereas the third limit, the software limit, is controlled by the program of the frequency converter. A typical software limit is 160% current for 60 seconds, after which the frequency converter reduces the load by lowering the motor frequency.
Normally, the hardware current limit is locked in the frequency converter already during manufacturing, but U.S. Pat. No. 4,525,660 discloses an over current circuit, in which the current limit is variable during operation. One input of a comparator here receives a current measuring signal from a current measured in the intermediary circuit or on the motor lines, the other input receives a current reference signal, which changes in dependence of the voltage-frequency ratio (U/f) of the frequency converter. When the current measuring signal exceeds the current reference, the comparator sends a signal in order to reduce the current or fully stop the frequency converter. The variable current reference signal consists of two contributions, a first fixed contribution being set by means of a potentiometer and a second contribution being determined as a function of the U/f ratio. The second contribution is found by means of a lookup table or a function calculation, the U/f ratio being the entry key and the current limit contribution increases with increasing U/f. Thus, the resulting current reference signal changes during operation of the frequency converter. However, this circuit has the disadvantage that the variation of the current limit is locked to the U/f ratio, which alone determines the characteristic, that is, the curve profile, of the current reference.
WO 97/36777 also describes a circuit for generating current references, which change during operation of a motor control, and the change takes place as a function of the speed of a vehicle. A current reference generator generates a hardware limit value by means of a table in a memory element comprising combinations of speed and current, while a microprocessor forms a software current limit. Via data lines, the microprocessor is connected with the memory element of the current reference generator, thus being able to change the profile of the hardware characteristic. Compared with U.S. Pat. No. 4,525,660, the circuit has the advantage that the current limit characteristic can be changed merely by changing the programming of the microprocessor. This gives the manufacturer a larger degree of freedom. Larger degrees of freedom are particularly desirable, when the manufacturer wants to make a general purpose motor drive, which can be used for different sizes of motors. A typical problem is the series-manufactured high-power frequency converter being connected to a low-power motor. The hardware current limit of the frequency converter is then locked and too high in relation to the motor. A further degree of freedom is desirable with regard to application areas, that is, areas in which the frequency converter is used. For example, HVAC (Heating, Ventilation, AirConditioning) and conveyor belt applications have different current limit profiles, but usually they are fixed in the frequency converter already during manufacturing. Thus, the frequency converter manufacturer is forced to have many variants, each having its particular hardware current limit.